1. Field of the Invention
The present invention relates to a probing test method of contacting a plurality of probes of a probe card with their corresponding pads on a chip on a semiconductor wafer so as to make it more reliable to electrically contact the probes with the pads.
2. Description of the Related Art
The probing test machine (which will be hereinafter referred to as prober) for conducting continuity test relative to a circuit on an IC chip on the semiconductor wafer includes loading/unloading, probing and test sections. The loading/unloading section is provided with an alignment stage. The probing section is provided with a wafer stage and a probe card. The test section is provided with a tester.
In order to automatically carry out the test by the wafer prober, the semiconductor wafer is pulled out of a cassette at the loading/unloading section, it is carried to the alignment stage and its pre-alignment, (or alignment of its orientation flat) is carried out on the alignment stage. After the pre-alignment it is carried onto the wafer stage at the probing section and pads each having a side of 60-100 .mu.m formed on the IC chip on it are aligned with the probes. After this relative alignment between the probes and the pads, current is applied to the probes while keeping the probes contacted with the pads, and signal is sent to the tester at the test section. Responsive to the test signal applied, the tester automatically finds whether the IC chip tested is a condemned one or not.
In the probing test, the position of the probe card is previously corrected relative to the wafer state in directions X, Y, Z and .theta., using a dummy wafer. Further, the arrangement of the pads on the IC chip is previously stored in a memory of the prober. The IC chip which is to become a product is probing-tasted on the basis of the memory information relating to this pad arrangement.
This probe card is attached to the underside of a test head and electric signal applied from the tester is sent to the probes through the test head. A fixed ring is attached to a base plate of the probe card and a plurality of the probes are vertically or obliquely planted on the underside of the fixed ring.
Each of the pads on the semiconductor wafer is usually made of metal such as aluminium, gold and solder alloy. This metal pad reacts with atmospheric air to form natural oxide film (such as aluminium oxide film), with which the pad surface is covered. This natural oxide film on the pad surface increases a contact resistance between the probe and the pad, thereby disturbing the transmission of test signal between the probe and the pad. It is therefore needed that contact resistance between the probe and the pad is reduced to enhance test accuracy.
Japanese Patent Disclosure Sho 64-35382 discloses a technique of controlling the lifting distance (moved amount for direction Z) of the wafer stage in such a way that the wafer stage can be overdriven in order to fully and electrically contact the vertical probes with their corresponding pads. According to this technique, the wafer stage is further lifted (or moved in the direction Z) only by a certain distance after the pads are passed over a height level (or reference level) at which they are only contacted with their corresponding probes. The pads which have been overdriven beyond the reference level are more strongly pushed against the probes, so that the natural oxide film on the pad surfaces can be broken by tips of the probes to thereby create full electric contact between the pads and the probes. However, it is extremely difficult to appropriately control the overdriven distance of the pads in the direction Z. Particularly, integrated degree of the chip became to 4 Mega, 16 Mega, 32 Mega-bits, contact surface of the pad is very small, and thickness of the pad is very thin. This causes the probes to stick into the pads themselves after breaking the oxide film on the pad surfaces, thereby forming scratches on the pad surfaces. These scratches will make wirings deficient at the following steps, thereby lowering the productivity. This becomes more remarkable as the circuit on the IC chip is more highly integrated.
When the wafer stage is overdriven beyond the reference level in the direction Z in a case where the probing test is conducted using the obliquely-planted probes, the tips of the probes bite into the pads themselves while sliding sideward on the pad surfaces, so that scratches can be formed on the pad surfaces. These scratches on the pad surfaces cause same demerits as described above.